The provision of sails for sailing vessels, and particularly those which race, presents the sailmaker with problems of not only the shape of the sails, but also the materials that are to be used, all in an effort to provide maximum speed under a wide variety of wind conditions. Not only is sail shape critical, rip resistance, UV life and the weight of the sails are all important factors in competitive sailboat racing.
As will be appreciated, in sailboat racing the disadvantage of a ripped sail computes to a minimum of 30 seconds for a sail change and can often result in the loss of a race and even the inability to remove the sail from the mast or forestay. Ripping or tearing of sails can also result in replacement with a non-optimal sail, making the boat less competitive.
Moreover, and especially with respect to upwind efficiency, the shape of the sail is critical. If the sail cannot maintain its originally-designed shape after initial wind gust loadings, a boat carrying this sail will also be rendered non-competitive.
Additionally, assuming the problems of rip resistance and shape have been solved, inertia effects make the boat carrying the lighter sails the most competitive.
All of the above militates towards the provision of new lightweight materials which are reinforced for strength and which are both flexible and foldable, while at the same time maintaining the designed shape.
Woven sailcloth has existed for many years as the preferred material in the fabrication of sails. Sailcloth in general involves the utilization of threads or strands in which fibers are twisted or spun together, often producing threads or strands with 1 thousand to 1 million yarns per strand. The thickness of these threads or strands is on the order of 4/1000 of an inch or 100 microns in diameter to 1/10 of an inch or 2500 microns in diameter. Whether woven or not, because of the relatively large diameter of threads or strands, making sails with threads or strands precludes improved performance mainly because the strength of the sails made in this fashion cannot be significantly increased. Nor can the weight of the sails be significantly decreased.
As is known, the shear strength of such a sailcloth or fabric is determined by the density or number of crossover points between warp yarns and weft yarns This is true regardless of whether the structure is woven or merely is the result of overlying of strand upon strand. The crossover density ratio is given by (diameter/spacing).sup.2. In a typical instance where ten mil strands or threads are utilized, with crossovers at 200 mil spacings, the density is a meager 1/400. This is an exceedingly low crossover density such that if such strands or threads are utilized to reinforce a laminate, for instance, of Mylar, the resulting material derives very little shear strength benefit from reinforcing strands. There the shear strength of the sail relies solely on the shear strength of the Mylar. It will be seen that a woven or non-woven structure of threads or strands cannot significantly improve shear strength of sails made in the traditional manner.
The reason, is that shear strength depends directly upon the number of crossovers, the points at which adhesive bonding attachment within the material is achieved. Low crossover densities determined by the relatively large diameter of the threads or strands in conventional sails makes a tight weave or lattice having large numbers of crossovers impossible.
It will be appreciated that were the strand or thread diameter to be reduced, for instance, to 5 microns, then the crossover density would increase by two orders of magnitude. Typically, aside from the weight reduction associated with providing very small diameter threads or strands, the crossover density could be improved over common thread or strand structures by a factor of 100 to 1 million if 5 micron diameter thread was available. However, such small diameter threads or strands do not exist.
As described in U. S. Pat. Nos. 4,679,519 and 4,708,080 to Linville and Conrad respectively, in order to provide a laminated sail material, non-woven strands or threads are lined up in a preferred direction, initially with no crossovers whatsoever. Thereafter, a second layer of strands is placed on top of these strands, with the second set of strands running in a different direction such that crossovers exist. However, even in this case the crossover density is exceedingly low due to the large diameter of the strands. Were the crossover density to be increased through the utilization of extremely small diameter strands, improved shear strength would result, with the desired goal being to achieve a shear strength ten times that of the Linville material.
Moreover, in terms of the specific Youngs' Modulus of the material with reinforcing elements reduced in diameter by 5 times and an optimally mixed hybrid of carbon and polymer monofilaments, such a sail material would be at least six times better than that presently producable.
It will, of course, be appreciated that with massive numbers of crossovers there would be exceptional rip resistance, and should the sail material start to tear it would abruptly stop, a point critical in sailboat racing, where materials are stretched to their outer limits.
The problem is one of finding a method of fabricating a reinforcing material in which the reinforcing elements are of exceptionally small diameter, while at the same time having increased or better yield strength and modulus. It is also important to provide increased crossover density through the use of newer materials, assuming that the crossover bonds can be made secure.
Even having developed such a light material with significantly increased shear strength, it is important for sails that the material not be deformed by stretching under load. As will be appreciated with all types of woven sails, since the yarns, when woven, are in an over-under zig-zag pattern in which the threads or yarns are bent across each other, when the woven structure is subjected to loading, the crossovers between the warps and the wefts are pulled so that the crossover is flattened at the overlap. What this means is that after the first major load is experienced by the material, the material does not return to its initially designed shape. The failure of the crossover to return to its original shape is referred to in the industry as the crimp problem in which the crossover stays flattened instead of returning to its original crimped over-under zig-zag shape. i.e., crimp causes a substantial nonlinearity in the stress-strain curve of the composite fabric. Hysteresis in the crimp relaxation process gives rise to permanent fabric stretch distorting the originally designed sail shape.
Since all sails are built with unloaded material, all sails built in this manner will lose shape under wind load. This causes sailmakers problems in the design of the sails because the amount of sail shape distortion is unpredictable. Because of this unpredictability, sails must be cut and recut after use to achieve the final desired shape.
The crimp problem is exacerbated the larger is the diameter of the threads or strands, due to the exaggerated over-under zig-zag of wefts and warps. Thus, because of the relatively large diameter of the strands or threads used in conventional sailmaking and the exaggerated over-under zig-zag or crimp, sail shape design is often a tedious cut-and-dry procedure.
Note, crimp-related problems are especially acute when dealing with large numbers of panels in a sail. Recutting of such sails after initial wind loading often requires recutting of each sail panel, a time consuming process which could be alleviated if crimp effects could be substantially eliminated.
In summary, crimp causes uncontrollable shape deterioration during the initial wind loads. This dramatically reduces the racing life of a sail and adds substantially to the trial and error sail design cycle. When a sail uses an assembly of a large number of specially oriented and shaped pieces, the crimp problem is even more severe due to the amount of recutting involved in achieving the desired sail shape. Thus, any sail material which reduces the magnitude of the over-under zig-zag or crimp is highly desirable. As will be seen, this can be accomplished by drastically reducing the diameter of the warp and weft-like reinforcing elements in a laminated material.
Another problem associated with laminated sails is the problem of voids. In the Linville system, with ordinary pressure lamination, substantial voids exist because of the gas which evolves during the lamination process. Typically three percent voids by volume reduces shear strength by 30 percent, which renders laminated sails devoid of the potential increased shear strength that can be achieved through reinforcing structures within the laminate.
By way of further background, while much of the art involved in making laminates comes from the aerospace industry in which carbon fiber structures and laminates are common, these structures and laminates are not well suited where flexible or foldable material is required. This is because the resin systems utilized are designed for rigid structures. Thus, carbon fiber masts and booms, and carbon fiber structural elements in aircraft have utilized manufacturing techniques for providing carbon elements in rigid epoxy matrices or binders. Such carbon fiber systems cannot be used where flexible and foldable material is required. Moreover, such systems are not readily adapted to sailmaking due to the unique dynamic conditions applying to the design of sails.